Paddock of dreams

At Currency Creek Arboretum, an hour south of Adelaide, over 32 hectares and the last three decades, Dean Nicolle has planted 10,600 individual eucalypt trees, representing eight Angophora species, 100 Corymbia and fully 872 Eucalyptus species or subspecies: some 97% of all known members of the Eucalypteae tribe.

It’s a taxonomic tapestry compressing 45 million years of evolution from across the continent into a single point in time and space. By attempting to grow every Australian eucalypt species in the one place, under uniform conditions, Nicolle has not only created the largest single collection of living eucalypts in the world, but a remarkable natural experiment.

Nicolle’s passion for eucalypts began when he was about eight years old. His dad gave him a copy of Ivan Holliday’s A Gardener’s Guide to Eucalypts and he read it from cover to cover, memorising every page.

“I loved that book,” he says. “All the pictures of all the different trees just fascinated me.”

As a teenager, Nicolle tracked down unusual eucalypts to plant on his parents’ four-hectare block on the edge of Adelaide’s southern suburbs. By the time he was 16, he’d decided on a plan.

“I’d like to be a eucalyptologist,” he told a newspaper at the time. “I now have about 170 species. My aim is to grow every eucalypt tree there is.”

It wasn’t long before he started running out of room.

In 1991, 17-year-old Nicolle went looking for one of the rarest species – Ramel’s mallee (Eucalyptus rameliana) – a plant he’d only ever seen depicted in a watercolour on the final page of Stan Kelly’s classic book Eucalypts.

This species had been collected by the explorer Ernest Giles on one of his journeys across the stony Gibson Desert in Western Australia. Giles delivered the specimen to the director of the Melbourne Botanic Gardens, Ferdinand von Mueller, who described it in Latin as trans montes, coming from the other side of the remote Alfred and Marie Range, although which side and how far depends, I suppose, on where you come from.

Before the trip, Nicolle and his dad trained by carrying 35kg backpacks full of bricks up hills near their home, and his parents hired a four-wheel-drive car and loaded it with camping equipment and supplies. In the days before GPS, walking into the desert meant keeping to a strict compass course and carrying a plastic jerrycan of water each. Nicolle and his dad walked for four days west across desert country while his mother and younger brother waited for them on the dusty track known as the Gunbarrel Highway. Every morning and evening they’d call in on a two-way radio, trying to work how far they had walked and how far
they still had to go.

They never found the elusive E. rameliana, (it was found later, in some abundance, 800km further west in the red sandy dunes). But Nicolle’s journey proved useful all the same. In preparation for their trip, Nicolle had asked to see the original specimen of E. rameliana in the Melbourne Herbarium, only to find that it was on loan to a botanist in Canberra. This led to an introduction to Ian Brooker, one of Australia’s leading experts in eucalypts, who became a mentor and colleague, encouraging Nicolle to study botany at university.

His undergraduate studies prompted Nicolle to take a more systematic approach. He’d grow four of each different species, subspecies or population. And so, in the early 1990s, he planted out the first block of the Currency Creek Arboretum. Every year since, a new block has been planted, telling the story of another collecting year – trees from the north, west, south and east.

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Paul Rymer started working with the Currency Creek Arboretum following a citizen science program. Rymer – an evolutionary ecologist at Western Sydney University’s Hawkesbury Institute for the Environment – and his colleagues were using genetic and physiological models to predict vulnerability to drought and to map die-back, but lacked the field observations to validate their predictions.

Prior to the devastating droughts of 2019–20, Belinda Medlyn set up the Dead Tree Detective to encourage people from across Australia to record and photograph the details of tree die-back events in their local environment. Nicolle got in touch.

Rymer and his colleagues realised that Nicolle’s experience of the drought at the arboretum had much more to tell them than just individual trees struggling.

“The issue with understanding susceptibility to drought,” Rymer says, “is being able to separate out each tree’s inherent sensitivity from their exposure to environmental impacts where the trees grow. Drought conditions vary, even across small areas, and so does the genetic composition of the trees … At Currency Creek you can separate these factors out, because all the trees of different species and different source populations have been planted in the same place. So we can measure the genetic differences among species side-by-side.”

Having thrived and diversified under climate instability, increasing aridity and fire frequency, eucalypts are probably better placed than humans to face an uncertain climate future. Their sclerophyllous leaves, regenerative habits and woody seed capsules have already adapted for the difficult times ahead.

The Currency Creek trees illustrate how important drought adaptations are. Perhaps unsurprisingly, the species that did best during the drought were those grown from seeds collected in hotter, drier climates than Currency Creek’s – particularly semi-arid mallee species. The ones that fared worst were from cooler and wetter climates.

Understanding drought resilience is more than just genetic variation. How that variation manifests in the tree’s phenotype is crucial. But mapping the functional traits – such as wood density, leaf size, shape and toughness – of widely dispersed tree species is onerous and time-consuming work. By planting so many species in the one area, Nicolle has done a lot of that leg-work for researchers.

“Instead of just looking at a dozen species,” says Rymer, “we’re looking at the drought response of almost all eucalypt species, and undertaking detailed investigation of the functional traits and genomics of 260 species.

“If we can maximise the genetic variation in the trees we plant, we can increase the likelihood of them surviving.”

This excerpt is republished online from Cosmos Magazine issue 93, which goes on sale on Thursday 2 December 2021.

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